This invention relates to a photoelectric smoke detector with a small-sized and thin-formed smoke-detecting unit.
A conventional photoelectric smoke detector, such as a scattered light smoke detector has a relatively bulky smoke detecting unit having a light emitting element and a photodetecting element, which is formed in a cylindrical shape having a relatively large diameter and a height. The smoke detecting unit is accommodated in a casing. The smoke detecting unit is comprised of the light emitting element and the photodetecting element, a base plate for mounting the elements thereon, and a peripheral wall provided so as to extend downward from the base plate, encasing the light emitting element and the photodetecting element therein. In general, the base plate and the peripheral wall are formed integrally with each other from a thermosetting synthetic resin.
The peripheral wall is comprised of wall elements which are planted circumferentially and allows smoke from the outside to enter but substantially prevents light from the outside from entering. The peripheral wall further contributes to substantial elimination of irregular reflection due to the internal configuration. The formation of the peripheral wall is what is called a labyrinth. If smoke enters inside of the peripheral wall, a portion of light emitted from the light emitting element is scattered by smoke particles. A fire detection output is generated when the scattered light is incident upon the photodetecting element and the output from the photodetecting element exceeds a threshold value indicative, for example, of a smoke density reached a danger level.
An insect net with is made of a perforated very thin metal plate to prevent possible intrusion by insects.
A conventional smoke detector of the type as described above, when installed on a ceiling of a room, is noticeably projected into the room, spoiling the scene. To solve this problem, there has been developed a scattered light smoke detector which has a small and thin smoke detecting unit having a reduced diameter and a height reduced to one third of the conventional detector height.
So far as the inventors know, this improved small and thin smoke detector is generally made of a thin material. For example, a base plate is made of a material as thin as 1mm or less. By this reason, a sufficient mechanical strength can not be ensured and, therefore, there may possibly be caused a problem that the base plate is inadvertently bent or, in the worst case, the base plate is broken. Besides, the wall elements are so fragile due to the thinned and small-sized configuration that the wall elements may possibly be bent if they are handled with special care, because the elements are not interconnected with each other.
In this connection, it is to be noted that the smoke detector of this type should clear a performance test subjecting the detector at atmosphere at a high temperature for a long time, for example 70.degree. C. for 21 days according to the UL standards. If the smoke detecting unit made of a thin material is allowed to stand in such atmosphere, the unit is liable to get deformed due to an internal stress caused in the shaping process.